1. Field of the Invention
This invention relates to an improved process for the manufacture of synthetic pipeline gas from carbonaceous fuel such as coal, lignite or the like. The importance of such a process has taken on new light in view of the energy crisis as pipeline gas has excellent heating value and may be used in the place of fuel oil. While there are known many processes for converting coal to pipeline gas, the need still exists for an economical and effective process.
2. Description of the Prior Art
One well known coal gasification process is the Institute of Gas Technology (IGT) HYGAS process which involves high pressure operations wherein hydrogen rich gases react with coal directly to form methane. While there are many coal gasification processes to which the present invention may be applied, it will be described in detail in connection with the IGT HYGAS process.
More particularly, in the HYGAS process, pulverized and pretreated bituminous coal is reacted at about 1200.degree.-1750.degree.F. and at a pressure of about 1000 to 1500 psig with hot, raw, hydrogen rich gas containing a substantial amount of steam. Before the coal is reacted, it is usually subjected to pretreatment since most bituminous coals tend to agglomerate and cake during hydrogasification. Such pretreatment usually consists of mild surface oxidation of the pulverized coal with air or oxygen in a fluidized bed reactor at about 800.degree.F. and atmospheric pressure. Although under the conditions of pretreatment most of the valuable reactive portion, i.e., volatile matter, of the coal is preserved, the pretreatment step still generates a substantial volume of low heating value fuel gas (on the order of 50 Btu per std. cu. ft.) containing a relatively high content of sulfur in the form of SO.sub. 2. Some tar and light oil are also formed. The loss of valuable carbonaceous components of the coal in this step reduces the overall thermal efficiency of the process. Moreover, the cleaning and upgrading of the low value fuel gas requires additional apparatus which adds to the cost of the process, particularly if environmental safeguards are maintained with respect to the quality of the fuel gas.
It has been proposed to use water as a slurry medium to facilitate the feeding of coal to high pressure gasifiers. However, water has a high heat of vaporization and in most instances there is insufficient heat available in a hydrogasifier effluent to evaporate the water. This has necessitated the use of a recycle oil with a low latent heat as the slurry medium which in turn requires troublesome separation and recycle.
An improved process is disclosed in U.S. Pat. No. 3,632,479 of Bernard Lee et al., which patent is specifically incorporated by reference herein. This patent discloses a process for the pretreatment of a caking coal to prevent agglomeration wherein an aqueous slurry of the coal is treated with air or oxygen at a temperature of about 500.degree. to 700.degree.F. and a pressure of about 1,000 to 3,000 psig in a treating vessel ahead of the gasifier. Off-gases from the treating vessel have to be separately disposed of and no consideration is given to the thermal requirements for vaporizing the water from the slurry which is then fed to the gasifier.